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Traditionally, vehicle routing plans are based on the deterministic information about demands, vehicle locations, and travel times on the roads. Especially, they consider that the travel times between nodes depend on distances. However, travel times fluctuate due to a variety of factors, such as accidents, traffic conditions, weather conditions, etc. Ignoring travel time variation can result in route plans that take the vehicles into congested urban traffic conditions.
The problem considered in this research is the time-dependent vehicle routing problem (TDVRP), that is an extension of the vehicle routing problem that has deterministic travel times between demand nodes. This problem is defined as follows. “A vehicle fleet of fixed capacities has to serve customers of fixed demands from a central depot. Customers must be assigned to vehicles and the vehicles routed so that the total time spent on the route is minimized. The travel time between two customers or a customer and depot depends on the distance between the points and the time of day. Time windows for serving the customers may also be given as well as a maximum allowable duration of each route (work day of the driver).”